The first tests for methane performed by Curiosity have come back negative. At least during the rover's first weeks on Mars, any methane present in the air above Gale Crater was at levels lower than five parts-per-billion. But the scientists studying the Martian atmosphere haven't come away disappointed. Detailed measurements of the isotopes present in the first samples indicate that the Martian atmosphere was once much thicker, which may help explain the past existence of liquid water on its surface.

The data come from Curiosity's Sample Analysis at Mars, or SAM instrument. Among other items, this contains a laser spectrometer, which can identify the chemicals present in the sample, and a mass spectrometer, which can identify their weights. The latter can also separate individual isotopes of elements. The data has also been cross-correlated; in a few cases (like CO2), the laser spectrometer can pick out the presence different isotopes, and check the ratios it obtains against those from the mass spectrometer. The readings were also tested against gasses trapped in a meteorite that was blasted off the surface of Mars and fell to Earth. So, we have a really good idea that all the instruments are operating well.

And, so far at least, those instruments are not picking up any methane. As of right now, the researchers operating the instrument put the upper limit on its presence at five parts-per-billion. They expect that limit to drop simply by statistically aggregating multiple measurements. That probably rules out much in the way of large, active sources on Mars at this moment, because it only takes about three months for Mars' atmosphere to evenly mix. But past evidence of methane suggests it might be released both locally and seasonally. So, the scientists are being quite cautious, and simply saying they haven't detected any methane within Gale Crater.

The current plan is to keep sampling, and see if anything turns up as the seasons change on the red planet. There is also a way of using the pumps that supply the gas to the instruments to selectively concentrate methane. That should increase the sensitivity by 10-fold.

So, what is present in Mars' atmosphere? About 96 percent of it is carbon dioxide, with two percent each of argon and nitrogen. Oxygen is only present at 0.14 percent, and carbon monoxide at 0.06 percent.

The arrival of winter on Mars freezes out about 35 percent of the atmosphere's CO2 at one of the poles, even as it's being liberated from the other. Depending on the precise timing, this could radically shift the relative amounts of most of the components of the atmosphere.

The instruments have also provided an indication that Mars' atmosphere has changed dramatically over time. We know what the probable isotope ratio of Mars' atmosphere was, based on measurements on Earth and other bodies. But the ratios no longer look much like that. In fact, the fraction of the heavier isotopes of argon is a full seven times what it is on Earth.

How can something like this happen? Some chemicals could have reacted with the Martian surface, but this isn't an option for argon, which is chemically inert. The only way to explain these isotopic differences is if Mars has been losing its atmosphere, a process that should preferentially result in the loss of lighter isotopes. This, in turn, indicates that Mars' atmosphere was once much thicker.

That's consistent with the extensive evidence of liquid water on Mars' surface (some obtained by Curiosity itself). If the heavier atmosphere was probably mostly CO2 and water vapor (as you might expect), the enhanced greenhouse warming would help raise the temperature, possibly enough to keep the water liquid.

Deviations between the isotope ratios of different elements may provide some hints of whether some chemicals are being refreshed from the interior of Mars, or are part of a larger chemical cycle. So nailing these isotope ratios down and looking for any long term changes may give us a better idea of whether there are any active processes reshaping the Martian atmosphere beyond the escape of gasses to space.

Promoted Comments

Because it didn't have the gravity to hold a CO2 and H2O atmosphere. If it it was bigger like Earth and Venus, or had an atmosphere composed of heavier, less volatile compounds, or if it was farther away from the Sun (lower temps decrease volatility) then it might have retained more of its atmosphere, like Titan has.

If a planet's gravity is low enough, gas molecules can escape simply by having a higher velocity (at a given temperature) than the escape velocity of the planet.

If there are no activities (such as volcanoes or water evaporation) that refresh the atmosphere, it will slowly lose atmosphere until it reaches equilibrium. That equilibrium again depends on the size of the planet and its gravity.

Heavier molecules such as CO2 are less likely to escape than lighter molecules such as O2, simply because their velocity will be lower at the same temperature. This is why there is relatively little hydrogen gas (the lightest molecule, H2) in the Earth's atmosphere, but giants like Jupiter and Saturn have atmospheres with large amounts of hydrogen.

If a planet's gravity is low enough, gas molecules can escape simply by having a higher velocity (at a given temperature) than the escape velocity of the planet.

If there are no activities (such as volcanoes or water evaporation) that refresh the atmosphere, it will slowly lose atmosphere until it reaches equilibrium. That equilibrium again depends on the size of the planet and its gravity.

Heavier molecules such as CO2 are less likely to escape than lighter molecules such as O2, simply because their velocity will be lower at the same temperature. This is why there is relatively little hydrogen gas (the lightest molecule, H2) in the Earth's atmosphere, but giants like Jupiter and Saturn have atmospheres with large amounts of hydrogen.

I've also heard speculation that the lack of an active geological core - and, importantly, it's resultant magnetic field - permits the atmoshphere to be more easily blown away by solar wind.

Why is measuring Methane important? What does its presence or lack of it suggest?

Potential metabolic processes. Because of the weak atmosphere and lack of a magnetic field, UV radiation should have decomposed any naturally occurring methane long ago. Something is emitting it, or it is escaping from the interior.

Well, finally! We have waited for the 1st sample long enough (a few weeks), and I was afraid the 2nd sample would be delayed too because of problems to understand the result. Maybe there was such a delay previously, but from here it seems the (seeming) absence of methane may have been controversial as well.

All is well that ends well. Atmospheric loss was predicted, and it will be interesting to see the papers modeling atmosphere over time.

Seems to me some oxygen is released by UV on ice as would be expected, the abundance is not matching the CO abundance. This is a good model for how Earth may have gotten its first, then highly toxic, oxygen into the atmosphere which may have kick started the split into the 3-4 major groups (counting the dsDNA megaviruses as in newer papers).

[By the way, seems the script on the previous methane post won't accept or directly allow my login. No biggie, it is the first time on Ars. (FWIW, I'm using Chrome at the moment.)]

Because it didn't have the gravity to hold a CO2 and H2O atmosphere. If it it was bigger like Earth and Venus, or had an atmosphere composed of heavier, less volatile compounds, or if it was farther away from the Sun (lower temps decrease volatility) then it might have retained more of its atmosphere, like Titan has.

Because it didn't have the gravity to hold a CO2 and H2O atmosphere. If it it was bigger like Earth and Venus, or had an atmosphere composed of heavier, less volatile compounds, or if it was farther away from the Sun (lower temps decrease volatility) then it might have retained more of its atmosphere, like Titan has.

It was my understanding that Mars' atmosphere had been blown away by the solar wind after it lost its magnetic field.

Mars looses atmosphere through Jeans escape, ie thermal loss: too high molecule velocity in the thermal tail to be held gravitationally, and solar wind loss, because the atmosphere is now too diluted to shield itself.

Likely the absence of magnetic field doesn't mean much, akin to how it means little for keeping Venus or even Earth atmosphere intact, as the link says.

Personally I'm *so tired* of the unsubstantiated "lack of magnetic field - oh, noes" hypothesis, as long as Venus and Titan has observably dense atmospheres that are retained over geological times. (Titan main loss is to methane being photochemically processed and raining down, not atmospheric loss.) It is a nice folk theory, but folk theories are not science theories.

The lack of magnetic field leads to mainly loss of hydrogen, which is a result of photolysis of water in the atmosphere. This is presumably how Venus, and the early Mars, lost most of their water. (The oxygen can be lost to space (the now hot Venus, likely) or to the crust and mantle minerals.)

But I have seen speculative papers on that the remaining local magnetic fields cooperates with the solar wind to sweep ionized gas away, increasing loss.

sonolumi wrote:

Why is measuring Methane important? What does its presence or lack of it suggest?

The whole idea with Curiosity is to look for habitability now or earlier, especially organics beyond the already verified water. Methane is used in metabolism and is a carbon source beyond what carbon dioxide and monoxide provides. It is also an intermediate of organics breakdown.

It is also indicative of active geological processes, either volcanism which provides heat, water and chemical energy for life on Earth or serpentinization of minerals which provides heat and is a chemical process that runs out of (preferably liquid, I think) water.

So both biotic and abiotic methane is highly interesting for looking at habitability and life.

Baeocystin wrote:

I'm surprised that there is a measurable oxygen content, even at a low percentage. What would be supplying it?

Ah yes, _that_ is the interesting question. If Mars had water in the atmosphere it would be dissociating high up. But it hasn't much water, so it could be UV on ice instead. At both poles, but especially the pure water ice pole.

This has been a suggestion for how Earth atmosphere once was oxygenated, which would have kicked in the splitting in 3-4 supergroups now seen. In some theories of metabolism this would be hard to understand otherwise.

Of course, it would switch around the whole oxygenating photosynthesis - oxygen atmosphere - global glaciation to global glaciation - oxygen atmosphere - oxygenating photosynthesis, so I don't know how likely it is. But Mars can now help sort these things out.

To follow up my original question, is Methane always a result of a biological process or can it occur on Mars as the result of other non-biological processes?

No; I believe one of Jupiter's or Saturn's moons have entire oceans of methane.

You can't say "no" here, the methane must be a result of _some_ process.

The methane atmosphere and, yes, hydrological cycle on Titan, one of Saturn's moons, is likely a result of methane release from a heavily ammonia soaked ice coated ocean. So there a chemical process is responsible. That ocean is akin to what is believed to be the case for Europa, for example.

"The arrival of winter..." is the writer claiming winter arrives over mars' entire surface all at once? or just in the crater where curiosity is now situated? thanks.

Though the orbital inclination (tilt) of Mars is somewhat similar to Earth (causing our seasons in the Northern and Southern hemispheres), Mars' eccentricity is higher, so I'd guess winter arrives for the entire planet at once.

"The arrival of winter..." is the writer claiming winter arrives over mars' entire surface all at once? or just in the crater where curiosity is now situated? thanks.

Though the orbital inclination (tilt) of Mars is somewhat similar to Earth (causing our seasons in the Northern and Southern hemispheres), Mars' eccentricity is higher, so I'd guess winter arrives for the entire planet at once.

IANAA, so someone please correct me if I am wrong.

"The arrival of winter on Mars freezes out about 35 percent of the atmosphere's CO2 at one of the poles, even as it's being liberated from the other"

full sentence indicates that as one pole freezes the other pole has an opposite reaction.

"The arrival of winter..." is the writer claiming winter arrives over mars' entire surface all at once? or just in the crater where curiosity is now situated? thanks.

Though the orbital inclination (tilt) of Mars is somewhat similar to Earth (causing our seasons in the Northern and Southern hemispheres), Mars' eccentricity is higher, so I'd guess winter arrives for the entire planet at once.

Mars doesn't have a protective magnetic field shielding the planet like we do here on earth. The magnetic field doesn't just protect us from radiation, it also protects the atmosphere from the solar wind. Without it the solar wind will strip away molecules from the atmosphere... sort of like sandblasting strips away paint or rust off metallic surfaces, but at an extremely slower rate. On a scale of many millions of years of exposure after Mars eventually lost its magnetic field.

This is all very sobering for those who keep insisting that all we have to do is terraform Mars and we have our Earth backup.

Evidence that Mars may have lost its atmosphere due to factors including lack of sufficient gravity, magnetosphere, or moon mass may be a sign that the conditions for Earth-like life support are far more complex and rare than a lot of people want to acknowledge.

Earth is still the one single place we know of where you can step outside of a sealed container, breathe in the atmosphere and take in a natural landscape with your own lungs and eyes, and live. While we do need to explore space, space is not going to be a viable backup any time soon. We still need to commit major resources to making sure we do not screw up Earth and maintain its environment as viable for as long as possible, because that's how long it's going to take to be able to move any significant percentage of humans somewhere else in the universe.

Because it didn't have the gravity to hold a CO2 and H2O atmosphere. If it it was bigger like Earth and Venus, or had an atmosphere composed of heavier, less volatile compounds, or if it was farther away from the Sun (lower temps decrease volatility) then it might have retained more of its atmosphere, like Titan has.

No. Mars should have enough gravity to hold an atmosphere. The problem is that without a large magnetic field to divert the solar wind, the atmosphere is literally blown away.

Personally I'm *so tired* of the unsubstantiated "lack of magnetic field - oh, noes" hypothesis, as long as Venus and Titan has observably dense atmospheres that are retained over geological times. (Titan main loss is to methane being photochemically processed and raining down, not atmospheric loss.) It is a nice folk theory, but folk theories are not science theories.

And I'm *so tired* of people quoting wikipedia without doing any independent research.

As for Venus, it is heavier than Mars and volcanic activity has replenished the atmosphere over time. Venus' surface was completely replaced through volcanic activity in the 300-500 million years ago timeframe, with volcanic activity continuing to today. That represents a lot of new material spewing into the atmosphere not very long ago. In spite of my over-simplifying in an earlier comment, obviously a heavier body will retain an atmosphere better. But Mars should be big enough to maintain more than its current whispy atmosphere had it been better protected from the solar wind. Clearly, though, over billions of years, Mars' smaller size would have allowed its atmosphere to escape more rapidly than what occured on either Venus or Earth. There's just good evidence that the solar wind dramatically speeds up the process when a magnetosphere is absent.

As far as Titan, it is much further from the Sun. Saturn is 5.6 times as far from the Sun as Mars is. So the effect of the solar wind will be 1/30th the effect on Mars. Furthermore, Titan orbits within Saturn's magnetosphere, so is partially protected from even that amount of the solar wind.

They used to claim that this was important, for climate say. But recently it was uncovered that the early orbital models were wrong. Mars, say, would have periods of ~ 0.5 billion years of modest variation in inclination, which is plenty of time for life (or intelligent life on other planets) to occur.

Chanur64 wrote:

The magnetic field doesn't just protect us from radiation, it also protects the atmosphere from the solar wind.

No, the magnetic field does diddly squat for protecting us against radiation. The CR infalling on Earth is efficiently blocked by the atmosphere. It is the _solar_ magnetic field that blocks ~ 90 % of the CR infalling on the system.

In some cases like CMEs, the magnetic field helps a little.

And as for protecting the atmosphere, read my longish comment - it doesn't, in general.

I am tired of this. Why are these folk physics models repeated as if they were tested science models!? We know that they tell a very little part of the history, at best.

BadSuperblock wrote:

This is all very sobering for those who keep insisting that all we have to do is terraform Mars and we have our Earth backup.

How so? If you study the models, they know this and build from the factual situation. We could give Mars an Earth equivalent atmosphere that would last ~ 1 billion years, which may be longer than our own current atmosphere will last. Our own atmosphere will lose CO2 to increased erosion as the Sun heats up in its old age and so lose complex life in ~ 0.5 billion years in some models.

Chuckstar wrote:

And I'm *so tired* of people quoting wikipedia without doing any independent research.

I quoted Wikipedia because it was simplest. I have studied the question rather thoroughly because it concern astrobiology. The question is open, as I noted, and the Wikipedia article is a good roundup of all that.

For every paper that claims any (usually non-thermal) loss process is the most important such you can find others claiming the reverse. The abstract (paywalled paper) of the paper you point to claims the reverse (modulo thermal vs non-thermal), the loss process is ~ 1/3 of the overall loss - today. To go from there to history of water loss is yet another question or two (since water isn't all of an atmosphere).

Independent research is a good start, but we have to study a question in depth. (Which, I confess, I haven't had time to here. I just know it is open.)

*** NOTE TO ADMIN ***

There is still login problems on some posts for some users (i.e. me), irregardless of browser. But the one I couldn't get to yesterday seems fixed, assuming the problem is at Ars end.

Because it didn't have the gravity to hold a CO2 and H2O atmosphere. If it it was bigger like Earth and Venus, or had an atmosphere composed of heavier, less volatile compounds, or if it was farther away from the Sun (lower temps decrease volatility) then it might have retained more of its atmosphere, like Titan has.

What I don't understand about this response is that if it is losing atmosphere, then it at one point had to have more, correct? Now, if it doesn't currently have the gravity to keep its atmosphere, then where did the gravity that was keeping the original atmosphere disappear to?

Hartman, I have no idea about how much oxygen was present to begin with, but what oxygen was there would have reacted with the minerals in the ground over time. Oxygen will oxidize metals and other compounds, and this process is what has given Mar's it's distinctive reddish color. This is why metal-heavy areas like the Grand Canyon have reddish/rusted hues in the rocks.

It's a slow process, but without a new source of oxygen to replenish what gets oxidized into the rocks and soil, over millions (or billions) of years it depletes the oxygen that was present.

There's also a second factor... CO2 is heavier than air and sinks here on earth. It would be my guess that this process concentrated the CO2 in the lower atmosphere, leaving Oxygen and other lighter elements more exposed in the upper atmosphere which, is the part that has been eroding away for some time via the solar wind and any gravitational losses.